This invention relates to the braking of a motor and its associated load in an AC motor drive. The motor drive uses an adjustable frequency control for at least a portion of the control of an AC motor. It is particularly adaptable to non-regenerative types of drives.
Variable frequency drives have often been used to vary the speed of an AC induction motor. Such drives can operate from a fixed frequency AC power available from an electrical utility to create variable frequency output power to the motor. Such drives can utilize power semiconductors controlled as ON or OFF switches to provide an adjustable speed control. Many of these drives can only accommodate power flow into the motor. As a result they cannot provide regenerative braking. Of the non-regenerative drives, many utilize a two-stage power conversion. The first stage converts AC input power to an intermediate DC source. The second stage uses semiconductor switches to act as an inverter converting the DC power to an adjustable frequency AC output. It is common that the second or output DC to AC conversion stage is capable of passing rated power in either direction. However, quite often the circuits used in the first or input AC to DC conversion stage are only capable of passing power in one direction, namely from the incoming AC line to the DC link output.
In many cases, the application of the motor drive requires occasional power flow in the opposite direction, for example to brake or decelerate a high inertia load. In such cases, it is common practice to add a power resistor and another semiconductor switch in a dynamic braking arrangement. In that arrangement the switch can connect the resistor across the DC link voltage to absorb the returning energy from the DC to AC conversion. Dynamic braking utilizes a resistor which absorbs the energy that has been stored in the motor and load inertia. The energy from the load is converted into heat in the resistor. Regenerative braking, using resistors, requires a high current switch which may be composed of semiconductors and a resistor of sufficient size to absorb the generated heat.
In other approaches where motor braking is required, the control can be designed to regenerate braking power, by feeding the power back into the AC incoming line. In such cases where there is an input AC to DC converter, the AC to DC converter can be designed using additional semiconductors switches to make the converter capable of passing power in both directions. However, this method is also more costly as it requires additional switching devices to handle high current. Because higher powered drives generally utilize three-phase current, the above methods of providing braking can require a larger number of switches since it is desired to balance all three phases. Such dynamic braking or regenerative braking can be costly because of the need for additional power circuit elements.
When the reverse power flow requirement is infrequent or modest, some drives have avoided these costs by utilizing the power supply output current at a zero frequency, in essence DC power to the motor windings. This creates a stationary magnetic field in the motor air gap. When the spinning rotor windings interact with this field, voltage is induced in the windings which causes rotor current to flow. The rotor currents in turn interact with the magnetic field to produce negative braking torque. Such an approach is sometimes called "DC injection braking." In drives where the control functions are performed in a microprocessor guided by software, DC injection adds no additional components and adds little to the cost of the basic drive. However, there are two specific drawbacks to DC injection braking. The first drawback is that the available torque at high speeds is quite low. This is due to the high slip in the motor and the consequential poor torque available per ampere. For example, if the injected DC current is limited to 100% of the motor rated current (to protect the drive), the torque produced at rated speed can be as low as 3% of the motor rated torque. The second drawback is that the control cannot estimate the motor's speed while DC injection is occurring; so that if a sudden return to forward torque is desired while the motor is spinning, a delay is needed to redetermine the motor speed before an accelerating adjustable frequency AC voltage can again be applied to the motor terminals. DC injection braking is, therefore, not desirable where high braking torque levels are required at or near normal operating speed, nor where it is desired only to brake the motor to a lower non-zero operating speed.
This invention to brake an AC electric motor relates to an electrical means to provide the braking torque as opposed to mechanical or friction braking. An object of the invention can be to provide an electrical braking scheme which can be to provide high levels of braking torque at rated speed. Another object of the invention can be to provide braking torque in non-regenerative converters, without the necessity of using additional costly semiconductor devices. Another object of the invention can be to provide a means to dissipate the braking energy as heat, without the need for dynamic braking resistors. The subject of the invention is to provide a high torque electrical braking method which can be utilized infrequently as a means to reduce the speed of the motor.
Certain preferred embodiments of the invention utilize at least one power conversion unit having an input AC to DC converter, and a DC to adjustable frequency AC output converter. One such type of drive is described in U.S. Pat. No. 5,625,545; and this patent is hereby incorporated by reference.
In some prior art test apparatus (for example Grantham et al.; "Dynamic Braking of Induction Motors", Journal of Electrical and Electronics Engineering Australia, Vol. 6, No. 3, September 1986) multiple frequencies are applied to an AC motor to simulate load during testing. Such test apparatus does not use the multiple frequencies injected to control the speed of a motor such as to brake the speed of a rotating rotor to a lower speed or stop.
Others, specifically Jansen et al in U.S. Pat. No. 5,729,113, utilize two frequencies in a PWM control for speed sensing and calculation.